Wall thickness and the presence of defects such as cracks are important factors in determining the fitness-for-service of structures such as above and below ground pipes and tanks, including bulk material and weldments. When a pipe is in operation, it can be subject to corrosion and/or erosion due to the content, flow and/or environmental conditions inside or outside of the pipe. Cracks can form and propagate due to the presence of manufacturing defects, creep, thermal cycling, fatigue and environmental conditions causing defects such as high temperature hydrogen attack (HTHA), stress corrosion cracking, etc. Corrosion and/or erosion results in the reduction in wall thickness, which can reach a point at which operating conditions becomes unsafe, considering that the pipe can be pressurized and may contain hazardous or flammable materials. Likewise formation and propagation of cracks, in welds for instance, can cause similar unsafe conditions. A failure may cause catastrophic consequences such as loss of life and environmental damage in addition to the loss of the use of the asset, and any corresponding costs associated with repair, loss of capacity and revenue loss.
Ultrasonic non-destructive evaluation techniques are commonly used for evaluating the integrity of industrial components. In the case of measuring wall thickness reduction due to erosion/corrosion, the traditional process involves using a portable handheld instrument and ultrasonic transducer (probe) to measure the wall thickness. The instrument excites the probe via an electrical pulse, and the probe, in turn, generates an ultrasonic pulse which is transmitted through an ultrasonic coupling material and then the structure. The probe also receives an echo of the ultrasonic pulse from the structure and through the ultrasonic coupling layer, and converts the pulse back into an electrical signal. The ultrasonic pulses that are transmitted into and received from a structure are used to determine the relative position of the surfaces (i.e. thickness) of the structure wall. More specifically, by knowing the travel time of the ultrasonic pulse from the outer wall to the inner wall and back (ΔT) and acoustic velocity (V) of the ultrasonic pulse through the material of the structure (through calibration or just initialization), a wall thickness (d) can be calculated—i.e. d=ΔT*V/2. There are many variants other variants of ultrasonic thickness gauging and flaw detection that are known to skilled practitioners of ultrasonic nondestructive evaluation.
These approaches require an operator to manually position a probe on the wall of the asset to take a reading. Not only does this necessitate the operator manually taking each reading, but also the measurement location must be accessible, which can be challenging and costly. For example, buried pipelines require excavation to access, insulated pipe requires costly removal of the insulation, and offshore assets require helicopter or boat access, and elevated vessels requiring scaffolding or crane access. While the measurement is relatively simple, the cost of access (scaffolding, excavation, insulation removal, etc.) is often much higher than the cost of measurement. Moreover, the operator is often subjected to hazardous conditions while taking the readings. Furthermore, to obtain trending data with thickness resolution of 0.001″ or better requires that the transducer be placed in the same exact location for consistent readings at regular time intervals. This is difficult and often impractical especially when the data-capture rate needs to be frequent. Variations in operator and/or equipment tend to skew the quality and integrity of the measurement data.
One approach for avoiding some of the aforementioned problems is to use installed sensors for asset-condition or -integrity measurement. The sensors are permanently or semi-permanently installed on the asset and can take advantage of features such as wireless data transmission to avoid costly wiring installations. Automated systems require no operator to be in the vicinity of the asset and can stream data to a control room or to an operator's desk. Current state of the art sensors have been shown to be useful and commercially successful for permanently monitoring structures using ultrasound.
A key challenge to implementing semi or permanently installed ultrasonic monitoring devices is the longevity of the acoustic coupling between the transducer and test piece. The ultrasonic coupling material is used to displace the air gap due to the imperfect mating of the transducer and test piece surfaces and further to provide a favorable ultrasonic propagation medium between the two in terms of loss, acoustic impedance, thickness, viscosity and so on. Traditionally, ultrasonic coupling materials comprise gel- or grease-like materials as seen in both medical and industrial ultrasonic applications. The requirements of these materials when subject to only temporary coupling are rather different than those required for permanent or semi-permanent use. When considering the harsh environments that the coupling agent may be subject to, for instance, high temperature, chemical exposure, moisture, radiation, etc., in addition to the rather long expected performance period of the coupling without maintenance (months to several years) the selection of such a material becomes quite challenging.
The problem of ultrasonic coupling has been investigated over the years as it relates to the problem of long term coupling of ultrasonic devices to structures of interest. Many solutions have been offered as optimum depending on the specific requirements of the test environment.
For example, US Patent application 20090110845 discloses the use of a thermoplastic film that is melted and used as an adhesive and coupling material. The thermoplastic material is used substantially below its melting point during operation and is only temporarily raised above its melting point in order to adhere the transducer and test specimen—essentially providing ultrasonic coupling while also adhering the transducer and test piece. While this method was shown to be effective, it has several drawbacks. First, the transducer and specimen must have their temperature raised substantially above their normal operating temperature in order to perform the adhesive joining. Second, the subsequent rigid bond is subject to stress and delamination over time of use. Third, the resulting bond does not allow moving or redeployment of the transducer if desired as the breaking of the adhesive bond will usually cause irreparable damage to the transducer.
Similarly, thermosetting materials such as epoxies have also been explored as options for installed ultrasonic sensors. While applying epoxies does not typically require the high processing temperature of a thermoplastic material as discussed above, the resulting rigid bond is still subject to failure due to loss of adhesion, particularly if strict surface coupling agent steps are not carried out. Additionally, the transducer is not movable and/or cannot be redeployed as the intentional breaking of the adhesion between transducer and test piece usually causes irreparable harm to the transducer.
U.S. Pat. No. 4,738,737 discloses a non-hardening grease composed of a heavy silicone fluid loaded with zinc oxide particles, such as is provided in Dow 340 heat sink compound. Applicants discovered this material after studying eighteen candidate materials for long term, high temperature ultrasonic coupling in nuclear reactor applications. While this heavy grease-like material shows promise for long term coupling, it can generally be moved by capillary forces during temperature cycling of the test piece. Generally, grease-like materials tend to separate, evaporate and/or outgas during operation that can limit the operational life of such materials. Along the same lines, U.S. Pat. No. 4,929,368 discloses a fluoroether grease acoustic couplant, such as DuPont Krytox grease, that is purported to be stable to 540 degrees Fahrenheit.
Other approaches involve the use of dry, metal to metal contact to achieve very high temperature capable (upwards of 900 degrees Fahrenheit) and long term stable ultrasonic coupling. These solutions tend to fulfil the need of high-temperature and long-term stability for installed sensor applications. Furthermore, the transducers are moveable. The downside of these approaches are the high coupling forces that are necessary to deform the transducer, test piece, and coupling foil and achieve basically a metal to metal seal in the acoustic path. To realize these solutions, substantial clamping apparatuses or welded-in-place brackets or studs are required. While this is certainly worth the effort for test pieces at extremely high operational temperatures, it is inconvenient and costly to implement.
Electromagnetic acoustic transducers have also been offered over the years as a solution for the ultrasonic coupling problem. EMATS directly excite the material surface via electromagnetic (Lorentz and/or Magnetostrictive) forces and therefore do not require coupling materials between transducer and test piece. While this solution is attractive for some applications, EMATS generally suffer from low sensitivity and ultrasonic resolution. The sensitivities may be 100 to 1000 times less sensitive than their piezoelectric counterparts.
It is recognized by Applicants that the ideal ultrasonic coupling agent is application dependent, and key factors include temperature range, reliability, long term stability and chemical compatibility. Accordingly, there is a need for an ultrasonic coupling for (semi) permanent application of ultrasonic transducers for the monitoring of assets such as those founding in the oil and gas industries with one or more of the following properties:                Provides the appropriate acoustic and mechanical properties to achieve ultrasonic coupling between an ultrasonic transducer and test piece.        Is chemically stable with minimum outgassing, noxious fumes and material degradation up to an elevated temperature in the range of 150 to 200 degrees centigrade.        Provides stable acoustic coupling at elevated and low temperatures and can withstand thermoscycling between the prescribed minimum and maximum operating temperatures.        Provides stable acoustic coupling for months or years as required by the maintenance intervals of the assets being tested.        Requires minimum or no special coupling agent of the transducer and test piece to achieve reliable results.        Has a low viscosity and/or can achieve thin coupling layers between transducer and test piece.        Is chemically compatible with the transducer and test piece materials.        Is removable, allowing repositioning of the transducer without harm to the transducer and/or test piece.        
The applicants have discovered a coupling agent that fulfills one or more of these needs, among others.